Image forming devices are known in which media having a layer of microcapsules containing a chromogenic material and a photohardenable or photosoftenable composition, and a developer, which may be in the same or a separate layer from the microcapsules, is image- wise exposed. In these devices, the microcapsules are ruptured, and an image is produced by the differential reaction of the chromogenic material and the developer. More specifically, in these image-forming devices, after exposure and rupture of the microcapsules, the ruptured microcapsules release a color-forming agent, whereupon the developer material reacts with the color-forming agent to form an image. The image formed can be viewed through a transparent support or a protective overcoat against a reflective white support as is taught in, for example, U.S. Pat. No. 5,783,353 and U.S. Publication No. 2002/0045121 A1. Typically, the microcapsules will include three sets of microcapsules sensitive respectively to red, green and blue light and containing cyan, magenta and yellow color formers, respectively, as taught in U.S. Pat. No. 4,772,541. Preferably a direct digital transmission imaging technique is employed using a modulated LED print head to expose the microcapsules.
Conventional arrangements for developing the image formed by exposure in these image-forming devices include using spring-loaded balls, micro wheels, micro rollers or rolling pins, and heat from a heat source is applied after this development step to accelerate development.
The photohardenable composition in at least one and possibly all three sets of microcapsules can be sensitized by a photo-initiator such as a cationic dye-borate complex as described in, for example, U.S. Pat. Nos. 4,772,541; 4,772,530; 4,800,149; 4,842,980; 4,865,942; 5,057,393; 5,100,755 and 5,783,353.
The above describes micro-encapsulation technology that combines micro-encapsulation with photo polymerization into a photographic coating to produce a continuous tone, digital imaging member. With regard to the media used in this technology, a substrate is coated with millions of light sensitive microcapsules, which contain either cyan, magenta or yellow image forming dyes (in leuco form). The media further comprises a monomer and the appropriate cyan, magenta or yellow photo initiator that absorb red, green or blue light respectively. Exposure to light, after the induction period is reached, induces polymerization.
When exposure is made, the photo-initiator absorbs light and initiates a polymerization reaction, converting the internal fluid (monomer) into polymer, which binds or traps leuco dye from escaping when pressure is applied.
With no exposure, microcapsules remain soft and are easily broken, permitting all of the contained dye to be expelled into a developer containing binder and developed which produces the maximum color available. With increasing exposure, an analog or continuous tone response occurs until the microcapsules are completely hardened, to thereby prevent any dye from escaping when pressure is applied.
Conventionally, as describe above, in order to develop the image, pressure is uniformly applied across the image. As a final fixing step, heat is applied to accelerate color development and to extract all un-reacted liquid from the microcapsules. This heating step also serves to assist in the development of available leuco dye for improved image stability. Generally, pressure ruptured capsules (unhardened) expel lueco dye into the developer matrix.
Approximately 100 mega Pascal or 14,500 psi normal pressure was required for capsule crushing as documented in prior art. This need for precise application of high pressure (high compressive forces) presented a limitation to the extensibility of the conventional imaging system. Small compact low cost printers typically employed micro-wheels or balls backed by springs and operate in a scanning stylus fashion by transversing the media. This allowed for low cost and relatively low spring force due to the small surface area that the ball or micro wheel (typically 2 to 3 mm diameter) contacted on the media. The disadvantage of this method was that the processing pitch required to ensure uniform development needs to be (approximately 1 mm for a 3/16″ diameter ball) which results in slow processing times for a typical print image format (4×6 inch). Ganging multiple ball stylus or micro wheels adds cost, and increases the possibility of processing failure due to debris caught under a ball surface.
Conventional high speed processing involved line processing utilizing large crushing rollers. To ensure the high pressure, (psi) required, these rollers tended to be large to minimize deflection. However, these large rollers were costly, heavy, and require high spring loading. Again, the extensibility of this method is limited as larger rollers (and spring loads) are required as media size increases.
Recent developments in media design (or the imaging member) as described in co-pending U.S. application Ser. No. 10/687,939 have changed the prior art structure of the imaging member within the context of the present invention to the point where the aforementioned means of processing are no longer robust. The use of a substantially non-compressible top clear polymer film layer and a rigid opaque backing layer which serves to contain the image forming layer of conventional media presented a processing position whereby balls, micro wheels or rollers could be used without processing artifacts such as scratch, banding, or dimensional or surface deformation. In addition, the non-compressibility of this prior art structure provided more tolerance to processing conditions.
The recent imaging member embodiment as described in the above-mentioned co-pending patent application, replaces the top and bottom structures of the media with highly elastic and compressible materials (gel SOC) (super over coat or top most clear gel comprising layer) and synthetic paper (polyolefin). The media as described in the above-mentioned co-pending application no longer survive these means of processing in a robust fashion where pressure is applied by a roller or ball. This is due to the fact that in the imaging member described in the co-pending application, the polyolefin paper backing that is used as fiber base substrates (cellulose fiber) present non uniform density, and the high compression forces required for processing in the conventional arrangements may make an “image” of the fiber pattern in the print, thus making the print corrupt.
It would be advantageous to provide a means or method of processing that did not invoke present methods utilizing high compression forces to provide a high quality image by improving the tonal scale development and density minimum formation of the imaging member. As mentioned, the need to provide a means of processing that will facilitate the use of the recently designed imaging member is needed. In addition, a processing means that would use plain paper as a substrate would be highly desired. Further, it would be advantageous to provide a means of processing that is low in cost, is fully extensible, and is mechanically simple and robust.
There is also a need for a processing arrangement that can compensate for unintentional thickness variations in media by being “self correcting” and being able to successfully process imaging members (media) that comprises low cost substrates including paper fiber supports.